The specification of each country's telephone communication system is somewhat different. As listed in Table 1, a worldwide telephone equipment manufacture may have to meet many different possible specifications in his products, e.g., a telephone. Usually, this problem is solved by adding additional I/O ports in the scanning circuits to detect additional functions that can be selected by a user of the telephone. As done in the prior art, each selection in Table 1 requires an additional I/O port which is tested to determine the state (i.e. selected or not selected) of each selection in Table 1. However, since the scanning circuits are often embodied in Integrated Circuits (ICs), the cost of the scanning circuits is primarily dependent upon the cost of the Integrated Circuits in which they are built. Generally speaking, packaging is the most expensive process among the processes involved in producing an IC. As listed in Table 2, the number of pins of an IC directly affects its cost. ICs with more pins are more expensive. Therefore, manufacturers try to reduce the number of I/O ports needed in a scanning circuit, to reduce the number of pins used by the IC in which it is embodied.
TABLE 1 ______________________________________ item spec. ______________________________________ dial rate select 10 PPS or 20 PPS make/break ratio 1:2 or 2:3 pause 2 sec. to 4 sec. flash 80 ms to 600 ms dialing pulse normal or n + 1 or 10 - n ______________________________________
TABLE 2 ______________________________________ pin packaging cost (cent) ______________________________________ 16 10.4 18 13.2 20 17.2 22 21.6 ______________________________________
A conventional keyboard for a telephone is shown in FIG. 1a. The keyboard has twelve input keys arranged in four rows by three columns. A basic keyboard scanning circuit for the keyboard is shown in FIG. 1b, where only one row port R1 and one column port C1 are shown for purposes of simplifying this example. Four clocks S1 to S4, shown in FIG. 1c, are used to drive the circuit. A common circuit for generating the clocks S1 to S4 is shown in FIG. 1d. At the beginning of a scanning cycle, S1 is at a high level, row port R1 is set to a high level with high impedance, and column port C1 is set to a low level with low impedance. If there is a key pressed down in row 1, then row port R1 assumes a low level. When clock S2 is at a high level, the status of the row port R1 is latched in latch 10A. When clock S3 goes to a high level, column port C1 is set to a high level with high impedance and row port R1 is set to a low level with low impedance. If the pressed key is on column 1, then column port C1 will assume a low level. When clock S4 is at a high level, the status of the column port C1 is stored in latch 10B. The scanning procedure described above repeats endlessly. By monitoring the status of the row ports and column ports, as stored in the latches, a determination can be easily made whether there is a key pressed and, if so, which key is pressed. The conventional keyboard scanning circuit scans the input keys of the keyboard; however, if detection of other selections is desired, e.g., selections listed in Table 1, additional scanning ports (I/O ports) are required. Thus, the cost of the conventional additional I/O ports.